JCM 800 loud cracking sound

[miketbass]

Hello all-
I have a Marshall 2204 Canadian model with a problem that I have been trying to track down for about a good solid month. It all started out of the blue....
After plugging in the amp fired right up and worked well through the low input jack. Upon switching to the high input jack the master volume was slowly brought up and after crossing about 4 a snapping, popping, crackling sound was heard. Intermittent sound would break through. I immediately dialed down the volume and began to cycle up towards the same setting of 4 and sure enough, it showed up again. Figured that a tube was on the fence and shortly after a smell of electricity was detectable, not a burning smell but the smell of a high voltage arcing (IME). I took the 800 home for inspection. Here is the long and the short of it.
1. Replaced all preamp tubes and fired up amp into a dead load. No problem was detectable. Took the amp down to our practice room to be able to crank it, and sure enough the problem reappeared. Once again the Low gain input works fine and I can wind the master up all the way. Figured power tubes were suspect even though these were a 6 month old pair. Amp eventually blew HT fuse.
2. Power tubes were replaced with a fresh set of Svetlana El-34s. Preamp section was replaced with the original lineup of 12AX7s (V1 - RFT V2 - Sylvania V3 - new production tung sol). A much closer inspection was given and sure enough, no evidence of burning or arcing was to be seen. I retensioned the power tube sockets and reflowed all the socket joints. Amp was run with a signal generator into a dead load and appeared to perform flawlessly for an extended period.
3. Amp passed test and was loaned to a friends band for a show. Played set without issue. This is 2 weeks ago.
4. Went in to record last weekend and set up in an isolation room. First take of first track went smoothly. Shut down amp and moved head into the console room leaving the cab in isolation. Amp was fired up and - you guessed it - a snapping, crackling sound was heard when the master was pushed past 4 or so. Keep in mind this is coming from the head not the cab. Its a very distinct sparking, almost a "ticking" sound. Seemed to react more to lower tones i.e. low E string. Amp will sit and idle with no signal and no issues, and low volumes seem not to induce the problem. When a big chord was hit the head would start snapping and crackling intermittently. Without wasting too much studio time I popped off the back plate and was able to see the EL-34 nearest the preamp tubes glow was pulsing with the crackle. Shut her down and brought her home.
I'm at a loss. I cannot get the snapping and crackling to happen here on my bench, running a 600hz signal at 600mv into a dead load. No issues. No parts appear to be failing. I've done fresh tubes although admittedly the phase inverter has not been changed. I know I have an intermittent connection or a short somewhere, I just cannot find it! I'm wondering what to do next. Could the OT be suspect? This problem only appears when the head is pushed hard but I'm pushing it hard into a dead load without issue. ANY suggestions or guidance would be appreciated. Thanks you guys ~

[txstrat]

I'd take an even closer look at the output tubes sockets and their surrounding parts (resistors, caps).
Can you play the amp with an open chassis (upside down) in a dark room (careful here, not too dark - you still need a bit of light not to touch the amps dangerous parts).
In a dark room you might see some arcing inside the amps chassis when you play it.
just my 2C

and welcome to the forum

[miketbass]

Thank you for the welcome. Is there any reasoning behind why I cannot get this issue to surface while running a dead load? Is it just an anomaly or is there something going on running a reactive speaker load that may change the factors? I have wondered if the positioning on the chassis upside down on my bench would cause a change in any intermittent issues. Thank you for your reply txstrat I may have a chance to do try your suggestion later this evening.

[txstrat]

reactive speaker load

+ OT reacting on the speaker load.

Just a thought

p.s. be careful working in the dark. tube amps are just as lethal with no light.

[Lee_ranaldo]

I had an issue with a super reverb months ago.

On the normal channel i had no problems but with the vibrato (bright switched on) a crackling, popping sound appeared past 3-4 in volume.

It was parasitic oscillation that saturates the output transformer and causes that kind of problems of intermitent operation. It sounded like i had a bad speaker or something

I don't know if that is your problem but if it was a solder join both channells would suffer from the same issue.

I'd suggest to change the filter caps and check some DC coupling caps with DC leakage on them.

This problem appears with certain notes under certain circunstances not all the time (that means if u play with low volume u may have no troubles at all).

[g1]

You said you were driving it hard into a load but then you said 600mV. What is your load and how much power are you putting in to it?

[miketbass]

Load is a 4 ohm 350 watt wire wound resistor.

[miketbass]

Is 600mV considered not a moderate input signal? I was more concerned about being able to wind up the volumes and did not spend too much time with my signal generator. I figured 600mv - 1V was probably a decent range to work with. Any guidance on this?

[g1]

Sorry, I thought you meant 600mV was your output into the load. What kind of power are you putting into the load?

[miketbass]

Input signal is a 600hz sine wave via Eico 377 signal generator. I clocked the output power into the dead load at about 60 watts before extreme clipping so I know that the amp at least can function well. The more I read about parasitic oscillation the more I'm beginning to believe that may be my problem. I have found a few accounts very similar to mine where the problem only surfaced when running into a reactive speaker load. All electrolytic caps were replaced roughly 6 years ago so I'm less suspect of an issue there. I'm wondering if lead dress has somehow become an issue as the problem is very temperamental, depending on where the amp is sitting or it's position. I think a chopstick and a good solid whack to induce the problem may be my best bet to track this issue down. I will post back with results.

[g1]

Just remembered a similar problem in an amp like that once. Turned out to be one of the lugs on a filter cap. Wiggling it with the chopstick (while putting power to load) finally made the problem appear. Somehow the internal connection in the cap to the lug was bad.
Another weird one was the ground connection to the output tubes cathodes, corrosion at the lug to chassis connection.
Also thinking maybe output transformer or a cracked pin in output tube socket.

[miketbass]

A very long overdue update....

After replacing tube sockets and really, really checking all of my solder joints the problem continued. It's been sitting for many months as I've been playing bass full time so I did not need the Marshall. I have been doing more amp repair lately and decided to dust off the 800 and give it another wack tonight. FINALLY after killing the lights to observe tubes/arcing I noticed a spark peeking through the grommet next to the power transformer. Could barely just see it and let it go a few more times, killed power, and took it back in the shop. Turns out that the massive fuse board in my Canadian JCM800 was the culprit.... one of the heater fuses was arcing to the chassis underneath the board. Why this just cropped up after many years, I have no idea. I mounted a fuse holder on top of the board and wired both wires to the top loaded connections. Popped the fuse in and put it through the paces. I'll be damned if it didn't work! I can't believe such a simple issue put me through so much grief. Many thanks to everyone that responded to my call for help and hey.... we were on the right track.